The present invention concerns a method and an apparatus for synthetic imaging.
The principle of synthetic imaging which is frequently also referred to as imaging with a synthetic aperture involves replacing the snapshot of an antenna or an objective with a large aperture by a multiplicity of chronologically successive recordings of a moved antenna or a moved objective with a small aperture or also with a multiplicity of chronologically successive recordings of a multiplicity of stationary antennas or stationary objectives with a small aperture.
The best-known system for synthetic imaging is the so-called synthetic aperture radar (in brief: SAR). In that case the transmitting and receiving antenna of a radar system which is mounted for example on aircraft are moved past an object. In the course of that movement the object is irradiated and suitably recorded, at variable viewing angles. If the path of the transmitting and receiving antennas is sufficiently known, the aperture of a large antenna can be synthesised from the intensity and the phase position of the high frequency signal emitted by the transmitting antenna and reflected back by the object to the receiving antenna, and thus a high level of positional resolution can be achieved in the direction of movement of the antenna. A specific synthetic antenna is calculated by means of the recorded data of the reflected radar signal, for each position irradiated by the transmitting antenna in the course of the flight therepast, the angular resolution of the synthetic antenna calculated at the azimuth being so selected that the geometrical resolution is the same in the direction of flight or movement, for all distances considered.
For stationary applications, for example for monitoring people by means of high frequency radiation in the megahertz and gigahertz frequency range, systems are known which, instead of a single pair of transmitting and receiving antennas which are in motion relative to the object, use a plurality of transmitting and receiving antennas which image the object at different angles and the signals of which are evaluated in accordance with the SAR principle. In that respect, either the transmitting antennas themselves or separate receiving antennas can be used for receiving the waves reflected from or transmitted by an object. To achieve spatial resolution which is as good as possible the signal emitted by a single transmitting antenna is received with a plurality of receiving antennas.
For that purpose the state of the art, for example DE 10 2005 016 106 A1, WO 2007/041024 or U.S. Pat. No. 3,909,827 discloses systems for and methods of imaging with a synthetic aperture, which distinguish from each other the signals emitted by the individual transmitting antennas after their reflection by an object or their transmission through an object, upon reception on a plurality of receivers. In that case the individual transmitting antennas emit their signals which are all at the same frequency in chronological sequence, that is to say signal emission from the individual transmitters takes place serially. In that method the signal received at each receiver can be uniquely associated with a transmitter at any time, but serial activation of the transmitters involves a comparatively long measurement time.